Modern combine harvesters can be used for harvesting and threshing a wide range of agricultural products. Combine harvesters comprise a threshing mechanism wherein the crop material is threshed in order to separate grains from the discardable part of the crop, and grain cleaning units, wherein the clean grain kernels are separated from other crop particles. In particular, in conventional combine harvesters for harvesting crop material, grain is threshed and separated in a threshing and separating mechanism and the separated grain, together with impurities of all sorts, such as chaff, dust, straw particles and tailings, is fed to a cleaning mechanism for cleaning. Clean grain is collected below the cleaning mechanism and fed to a grain tank for temporary storage. The tailings are separated from the clean grain and impurities by means of sieves and provisions are taken for recycling the tailings through the combine harvester for reprocessing. This reprocessing involves either recycling the tailings through the threshing and separating mechanism and/or treating them in a separate tailings rethresher means.
Cleaning sections are operating under a wide range of conditions, which sometimes result in a temporary overload of the sieve sections. Cleaning units in combine harvesters may be temporarily heavily disturbed by local field and crop conditions, such as for instance rapid slope variations or an abrupt increase in crop throughput when the harvester is driven from a low yield spot into a zone with higher yields. Disturbance may also be caused by wrong separation or cleaning settings, difficult cleaning conditions (e.g., from a large amount of green material), or when the threshing settings of the combine harvester are adapted. These transient disturbing effects can result in a sudden overload of the cleaning section, whereby a heap of crop material accumulates locally on the upper sieve of the cleaning unit such that the cleaning unit can not fulfill its function. Moreover, a constant overload of the cleaning mechanism ultimately may lead to a substantial rise of the tailings flow ending up with excessive cleaning losses and finally also with blockages of the tailings return system.
Commonly, the operator attempts to reduce sieve losses in such cases by ground speed control strategies, in particular by reducing the ground speed. Where the ground speed is controlled by an automatic controller in response to a grain loss signal, such controller will decrease the crop throughput set-point drastically in response to the sieve overload effect, resulting in a minimal ground speed. However, it is not useful to apply ground speed variations once a heap of crop material is present on the upper sieve section because it takes a considerable amount of time to recover from the transient effect, such that a significant amount of crop will be lost before the effects of the new speed have stabilized. Furthermore, the sudden increase of sieve losses disturbs the closed loop behavior of automatic grain loss control algorithms and causes serious discomfort to the operator, when operating in automatic ground speed control mode. No operator supports very well an ongoing succession of accelerations and decelerations of his machine.
It is therefore a general object of the present invention to provide a method for optimizing the operation of a grain cleaning system in a combine harvester during a disturbance in the grain cleaning system. It is in particular an object of the present invention to provide an improved method for reducing sieve losses due to a temporary overload of the upper sieve section in a grain cleaning system.
The present invention also aims to provide an improved grain loss control algorithms which is capable of activating effective sieve control actions in reply of a disturbance in the grain cleaning system.